1. Field of the Invention
The present invention relates to a method for producing an electronic part mounting structure in which electronic parts such as IC packages are electrically connected to the surface of a printed circuit board by using low-melting point metal, More particularly, the present invention relates to a method for producing an electronic part mounting structure capable of sufficiently and assuredly supplying solder to a portion between the terminal of a printed circuit board and the leads of an electric part while maintaining a predetermined thickness required to establish a connection between the printed circuit board and the electronic part.
2. Related Art Statement
Recently, in order to meet a desire to improve the function of a multiterminal IC package and that raise the density of the same, a QFP (a Quad Flat Package: a four directional flat package) having about 200 pins or less has been used in place of an SOP (a Small Out-line Package: a two-way flat package) the number of pins of which is less than about 32. Furthermore, in a case where the number of the pins is increased to a number of 200 pins or more which exceeds the limit of the number of pins which can be provided for the QFP, a TAB (Tape Automated Bonding) has been used. Furthermore, the shape of the IC package has been changed from an insertion mounting method arranged such that an external lead of an IC package is inserted into a through hole formed in a printed circuit board before they are connected to each other by soldering to a surface mounting method in which the lead is directly connected to the surface of the printed circuit board and which thereby exhibits an excellent mounting efficiency.
The above-described conventional IC package of the narrow-pitch and multi-pin surface mounting type must maintain the flatness of its lead at a predetermined value (for example, 0.1 mm or less) by correcting the bend of the lead in order to secure the reliability of the LSI. In order to achieve this, a conventional method has been arranged in such a manner that each lead is held at the time of mounting the electronic parts on the printed circuit board or another method has been employed which is arranged in such a manner that soldering is performed while holding the lead with a load applied to the package at the time of reflow-connecting the solder. However, according to either of the above-described methods, the interval between a lead 4 of an electronic part 3 and a terminal 2 placed on the top surface of a printed circuit board 1 is narrowed excessively as shown in FIGS. 18 and 19. What is even worse, molten solder 6 undesirably overflows the junction while leaving a poor quantity of the solder 6 in the end portion of the junction, causing a problem to arise in that the quantity of the solder 6 present in the junction is insufficient to establish the connection. As a result, the junction is undesirably constituted by hard and brittle Cu-Sn alloy layers 7 and 8 composed of Sn contained in the solder 6 and Cu which is the base material of each of the lead 4 and the terminal 2. Therefore, there arises a problem in that the solder 6 cannot exhibit desired strength and thereby the reliability of the junction excessively deteriorates because impurities or Pb components contained in the solder 6 are deposited in an interface 9 between the alloy layers 7 and 8.
In order to overcome the above-described problem, a structure has been disclosed in, for example, Japanese Patent Unexamined Publication No. 60-163493 in which the lead terminal has a portion projecting toward the soldering land of the printed circuit board while forming a wedge shape to create a non-parallel gap between the lead terminal and the soldering land so as to maintain the quantity of the solder required to connect the above-described non-parallel gap. Another structure has been disclosed in, for example, Japanese Patent Unexamined Publication No. 63-315261 in which, in a case where a flexible printed board (FPC) for a thermal recording head and a substrate having a different coefficient of thermal expansion are connected to each other by thermal-pressure soldering, dummy terminals acting to adjust the distance between the above-described two elements and formed by the thick film process are provided between the same at a predetermined interval so as to, in the portion to be soldered, form a columnar portion capable of securing a height required to perform soldering.
However, although it has been said that the structure, in which the lead terminal is bent to form the wedge shape, enables the necessity of holding the lead to be eliminated and as well as a desired effect can be obtained even if the quantity of the solder supplied deviates from a predetermined quantity, the degree of bending to form the above-described wedge shape is restricted by the interval between the soldering lands formed on the printed circuit board. Furthermore, the quantity of the solder to be maintained at the non-parallel gap cannot be made constant depending upon the degree of bending to form the wedge shape. Therefore, a stable connection cannot be established by soldering. On the other hand, it is said that the structure, in which the above-described dummy terminals are formed, has an advantage in that the dummy terminals act as spacers at the time of performing heat-pressing by using a hot ram and are arranged to be disposed at proper intervals in accordance with the length to be soldered. However, if the intervals of the positions of the dummy terminals are too long, the FPC can easily be deformed, causing a problem to take place in that the height of the solder formed in each junction realized by soldering cannot be made uniform. On the contrary, if the same are too narrow, a problem takes place in that the intervals between the junctions realized by soldering becomes too wide because the number of the dummy terminals increases excessively although the above-described deformation of the FPC can be prevented. As described above, there arises the problem in that the height realized by soldering cannot be made uniform because the height excessively depends upon the intervals between the dummy terminals. Therefore, there is a technological desire to maintain the height of the solder at a predetermined height.